1. Field of the Invention
This invention relates to a controller for a switch-mode power converter and the control method therefor.
2. Brief Description of the Prior Art
A switch-mode power converter is a multiport system which receives electrical energy at an input port and transforms this energy using a network of switches and reactive elements (the power train) to supply electrical energy at desirable voltage levels to one or more output ports. A feedback control system is connected between one of the output ports (the controlled output) and one or more switches in the power train. This feedback system senses the voltage (and possibly the current) at the controlled output port and adjusts the duty cycle (and possibly the period) of the switches in the power train to maintain the desired conditions at the controlled output port. The remaining output ports, if any, are slaved to the controlled port and generate voltages or currents in definite ratio to the controlled port.
The simplest class of converters (canonical converters) has one input and one output port and utilizes a power train having one inductor, one capacitor and two switches, one of the switches being customarily implemented using a diode. The three canonical converters are the buck converter, which generates an output voltage lower than the input voltage, the boost converter, which generates an output voltage higher than the input voltage and the buck-boost converter, which generates an inverted polarity voltage whose magnitude may be greater than, less than or equal to the input voltage.
Prior art converters have used numerous methods to control the switches to regulate the flow of energy through the power train. Since most switch-mode power supplies are designed to provide definite output voltages, the output voltage at the controlled port is almost always a controlled variable. Control schemes have been developed which also include output current, inductor current, switch current, input voltage, etc. as control variables. Most control methods require the addition of lead-lag compensation networks to adjust the phase shift around the feedback loop to ensure stability. Most implementations of these control methods include one or more operational amplifiers to provide gain and to allow convenient implementation of the lead-lag networks. The presence of operational amplifiers in the feedback path is undesirable from for at least two reasons, these being (1) that these amplifiers require considerable supply current to operate, which may degrade the efficiency of low-power converters, and (2) the amplifiers always inject some additional (and undesirable) phase shift at higher frequencies, making it necessary to apply more complicated lead-lag compensation networks than might otherwise be necessary.
Voltage-mode control schemes normally require a ramp generator, a comparator and an operational amplifier. The ramp generator produces a sawtooth or triangle voltage waveform having a convenient amplitude, for example, one volt. The operational amplifier compares the output voltage of the converter against a reference voltage and generates an error signal proportional to the difference between the output voltage and the reference voltage. The comparator then compares the output of the amplifier to the ramp. Whenever the error signal is larger than the ramp, the comparative activates switches in the power train which increase the current flowing through the converter. Whenever the error signal is smaller than the ramp, the comparator deactivates the switches and allows the current flowing through the converter to decrease. The comparator and ramp generator together comprise the pulse-width modulator (PWM) circuit used to transform the error voltage into a digital control signal used to drive the switches.
A prior art control scheme known as direct-summing voltage-mode control eliminates the need for the operational amplifier and its associated lead-lag compensation network by using a ramp having a very small magnitude, for example, 50 to 100 mV. Because of the small magnitude of the ramp, the circuit no longer requires large amounts of amplification. If the operational amplifier is removed, the gain of the resulting circuit will still be adequate to maintain acceptable line and load regulation. Since the amplifier no longer contributes excess phase, the power train of the buck controller now behaves as a two-pole system. A zero can be introduced into the transfer function of the control loop by placing a small resistance in series with the output filter capacitor. By this means, the circuit can be stabilized without requiring a lead-lag network. A similar stabilization scheme can also be used for boost and buck-boost controllers as long as the currents through the converter do not exceed several hundred milliamperes. At higher currents, the right-half-plane zero caused by the interruption of output current flow by the switch moves to sufficiently low frequencies to require a dominant-pole compensation network.
Prior art direct-summing voltage-mode controllers have several disadvantages. First, it is difficult to derive a low-amplitude ramp having the necessary fidelity. Noise coupling from the power train can cause glitches which interfere with the proper operation of the pulse-width modulator. Higher gains require smaller signals which are even more nose-sensitive. Second, the ramp voltage waveform used in a direct-summing controller must include a small ramp voltage imposed on a larger fixed offset voltage. For example, the ramp may consist of a 50 mV sawtooth waveform imposed on a 1 volt DC offset. Although it is possible to generate a waveform of this type using capacitive coupling between a low-amplitude sawtooth ramp generator and a resistive bias network, the impedance of the biasing network renders this solution extremely vulnerable to noise coupling. The conventional solution has been the substitution of a summing comparator for the single-input-port comparators used in conventional PWM modulators. A simple summing comparator has two input ports and asserts its output if and only if the sum of the differential voltages across both input ports exceeds zero. The use of multiple input ports eliminates the need for a DC offset on the ramp waveform. Unfortunately, the summing comparator generally requires several times more current than a conventional single-input-port comparator and has propagation delays which are several times as long as those of the single-input-port comparator. As a result, summing comparators make it difficult to construct high-speed low-overhead-current control circuits of the sort required for many modern portable electronic devices.